The Wnt family of secretory glycoproteins has important roles in regulation of a wide range of biological and pathophysiological processes that include tumorigenesis. The canonical Wnt signaling pathway is initiated by the binding of canonical Wnts to their receptor complexes consisting of the LDL receptor-related protein (LRP) 5/6 and frizzled (Fz) proteins and negatively regulated by many naturally occurring antagonists including the Dickkopf (Dkk) family of polypeptides. Through a cascade of events, ?-catenin is stabilized and forms a complex with the TCF/LEF-1 transcription factors to activate gene transcription. Aberrant canonical Wnt signaling has long been known to play a key role in colon cancer formation because mutations in the Wnt signaling components, frequently APC and less frequently ?-catenin and Axin, are associated with most of colorectal malignancies. In addition, recent evidence has implicated Wnt signaling in the formation and metastasis of prostate cancer. Although studies have suggested that extracellular Wnt proteins, acting in either autocrine or paracrine manner, may have an important role in the growth of colorectal tumor cells that even contain APC or ?-catenin mutations, their roles in tumor development and metastasis in vivo have not been well studied. The redundant expression of Wnt genes in most of the tissues makes it difficult to address this question. In this study, we propose to use a combination of the chemical genomic and genetic approach to test the hypothesis that extracellular Wnts have an important role in colorectal and prostate tumorigenesis. In our previous effort of screening small molecular inhibitors for Dkk using a virtual screening approach that combine structural biology, computation modeling, and biological assays, we have serendipitously identified small molecule Wnt inhibitors. In our preliminary studies, we found that the Wnt inhibitor can inhibit colon cancer cell growth and tumor formation in the colorectal cancer mouse model APC(min) mice. In this proposal, we will: 1) Investigate the role of extracellular Wnt stimulation in tumorigenesis and metastasis by using the Wnt antagonistic compounds in colorectal cancer mouse model and culture systems in combination of a genetic approach---the temporally regulated transgenesis. 2) To further characterize the Wnt antagonistic compounds and to refine and expand the virtual screen. Project Narrative: This study investigates the mechanism by which colorectal cancer formation, a prevalent cancer in developed countries. This study will not only provide better understanding the etiology of the disease, but may also yield novel therapeutic targets and agents for treating this disease.